:h manpager describes how to use vim as a manpager:

Works on:

  - Linux
  - Mac OS
  - FreeBSD
  - Cygwin
  - Win 10 under Bash

Untested:

  - Amiga OS
  - BeOS
  - OS/2

For bash,zsh,ksh or dash by adding to the config file (.bashrc,.zshrc, ...)

    export MANPAGER="env MAN_PN=1 vim -M +MANPAGER -"

The line above works for me on Linux (Debian 9.2), but the same setting on a Mac (10.11 El Capitan) causes the following error every time I invoke man:

Error detected while processing function <SNR>57_MANPAGER:
line   16:
E684: list index out of range: 2
E15: Invalid expression: page_sec[2] page_sec[1]
Press ENTER or type command to continue

Question 1: Is this a bug?

Should I file a bug report with vim?

EDIT: This appears to be a bug in the $VIMRUNTIME/plugin/manpager.vim plugin. Line 18 reads:

let manpage = matchstr( getline(1), '^' . pagesec_pattern )

It can be fixed by replacing getline(1) with getline(nextnonblank(1)).

Question 2: Is there a workaround?

I've found alternate settings for the MANPAGER environment variable on /r/vim and this StackExchange that make it work on macOS, but both of them cause navigation via <C-]> to break and give the folllowing output:

sh: -c: line 0: syntax error near unexpected token `||'
sh: -c: line 0: `(cd '/usr/share/man' && (echo ".ll 12.1i"; echo ".nr LL 12.1i"; /bin/cat '/usr/share/man/man1/vimtutor.1') | /usr/bin/tbl | /usr/bin/groff -Wall -mtty-char -Tascii -mandoc -c | ( || true))'
Error executing formatting or display command.
System command (cd '/usr/share/man' && (echo ".ll 12.1i"; echo ".nr LL 12.1i"; /bin/cat '/usr/share/man/man1/vimtutor.1') | /usr/bin/tbl | /usr/bin/groff -Wall -mtty-char -Tascii -mandoc -c | ( || true)) exited with status 512.
No entry for vimtutor in section 1 of the manual

EDIT: Even with the fix described above, I'm continuing to get this error when navigating between man pages. It looks like it's being caused by the ( || true) statement in the shell command, but I'm not sure where that shell command is being invoked from.

Does anyone know of a fully working solution?

Running vim 8.0.596

$ vim --version
VIM - Vi IMproved 8.0 (2016 Sep 12, compiled May  8 2017 15:44:54)
MacOS X (unix) version
Included patches: 1-596